1. Field of the Invention
The present invention relates to an AC/DC converter for rectifying a commercial AC power supply voltage or converting a commercial AC power supply voltage to a stable DC voltage and, in particular, to an AC/DC converter for use in a transformerless power supply of the industrial consumer field.
2. Description of the Related Art
HV-2045E of HARRIS has been placed on the market as an AC/DC converter for use in a transformerless power supply. Its detailed specifications and circuit arrangement are shown in File Number 2487 of the brochure of HV-2045E.
The AC/DC converter of HV-2045E includes a double-gate thyristor serving as a main switching device. A commercial AC power supply voltage (hereinafter referred to as AC power supply voltage) is applied to the double-gate thyristor. When the AC power supply voltage exceeds a predetermined value of about 3 V.sub.F, the thyristor is turned on, and this turn-on state is maintained during a period of time corresponding to a positive sinusoidal half wave of the AC power supply voltage. During a period of time corresponding to a negative sinusoidal half wave thereof, the current flowing through the thyristor is smaller than a holding current, and the thyristor is turned off, thereby maintaining this turn-off state.
Since, however, the thyristor serving as a main switching device of the AC/DC converter is of a latch type, there is a possibility that the AC/DC converter cannot be regulated if a current flowing through a load is increased suddenly at a frequency which is higher than a commercial frequency. More specifically, since the thyristor can be turned on only once during the period of time corresponding to the positive sinusoidal half wave, if the load varies (becomes great) immediately after the thyristor is turned off, an output voltage of the AC/DC converter will be decreased, even though it is backed up by electric charge of a capacitor. The decrease in the output voltage prevents the AC/DC converter from being regulated.